The present invention relates to a hydraulic fluid pressure control system, included in an automatic transmission, for controlling the shifting operation of a gear transmission mechanism which includes a plurality of friction engaging devices, and more particularly relates to such a hydraulic fluid pressure control system, in which a shift timing control subsystem is provided for ensuring good control of the timing of shifting between two adjacent speed stages of the gear transmission mechanism, each of said two speed stages being provided by engagement of a particular friction engaging device corresponding thereto.
Various different types of gear transmission mechanism are used in automatic transmissions. In these gear transmission mechanisms typically each of the speed stages is provided by supplying actuating hydraulic fluid pressure to a particular hydraulic pressure actuated friction engaging device such as a hydraulic clutch or brake, and shifting between two speed stages is performed in such a manner that a certain first friction engaging device which has been engaged by supply of a hydraulic fluid pressure thereto to provide e.g. a lower speed stage is disengaged by exhausting of the hydraulic fluid pressure therefrom, while a certain second friction engaging device which has been disengaged is engaged to provide a higher speed stage by supply of a hydraulic fluid pressure thereto.
For the shifting of a gear transmission mechanism between two speed stages, various hydraulic fluid pressure control systems have been proposed in the prior art. Typically, such a hydraulic pressure control system includes a lower/higher speed stage shift valve having a valve element, which is moved axially to and fro by the balance between a throttle hydraulic fluid pressure proportional to the amount of accelerator pedal depression, and a governor hydraulic fluid pressure proportional to the current road speed of the vehicle, so as to connect a pressure chamber of a first friction engaging device to a line pressure source while connecting a pressure chamber of a second friction engaging device to a drain passage when its valve element is shifted to one axial position by the throttle pressure prevailing over the governor pressure, whereas, on the other hand, when the valve element is shifted to another axial position by the governor pressure prevailing over the throttle pressure, so as to connect the pressure chamber of the first friction engaging device to a drain passage while connecting the pressure chamber of the second friction engaging device to the line pressure source.
In order to control a proper timing of the engagement and disengagement and disengagement of the first and the second friction engaging devices, such a speed stage shifting system is generally accompanied by a shift timing control subsystem, such as an upshift timing control subsystem, which controls the timing of the disengagement of the first friction engaging devices in relation to the engagement of the second friction engaging device during the upshift transmission. Such a shift timing control subsystem operates by regulating the flow resistance of the passage by which the actuating pressure chamber of the friction engaging device to be disengaged is drained. The shift timing control subsystem includes a variable throttling means located at an intermediate point along the drain passage for the pressure chamber of the first friction engaging device, and is responsive to the pressure magnitude of the fluid pressure supplied to the pressure chamber of the second friction engaging device, and, during the first part of this shifting process, when the pressure value in said second friction engaging device pressure chamber is below a certain threshold value at which substantial torque transmission between the friction engaging members of said second friction engaging device occurs, causes said drain passage for the pressure chamber of the first friction engaging device to have a relatively high flow resistance, so that during this first stage of engagement of the second friction engaging device the first friction engaging device is being relatively slowly disengaged, since the pressure in the pressure chamber thereof is only being vented relatively slowly past said relatively high resistance presented by said shift timing control subsystem. On the other hand, when the pressure value in said second friction engaging device pressure chamber rises to become above said threshold value, so that now definitely substantial torque transmission between the friction engaging members of said second friction engaging device is occurring, then the shift timing control subsystem causes said drain passage for the pressure chamber of the first friction engaging device to have a relatively low flow resistance, so that thereafter during the latter stage of engagement of the second friction engaging device the first friction engaging device is being relatively quickly disengaged, since the pressure in the pressure chamber thereof is now being relatively quickly vented past said relatively low resistance now being presented by said shift timing control subsystem. Typically, such a shift timing control subsystem comprises a shift timing valve, through which the pressure chamber of the first friction engaging device is drained to one or another drain port with various throttling devices being provided en route, or the like, and to which the actuating pressure for the pressure chamber of the second friction engaging device is supplied as a control pressure for moving the valve element thereof. This type of shift timing control subsystem is per se well known in the transmission art.
However, a problem which has arisen with such prior art transmission control systems is that, since typically the control port of the shift timing valve is supplied with the actuating hydraulic fluid pressure which is being supplied to the second friction engaging device from an intermediate supply point in the hydraulic fluid conduit which leads from the lower/higher shift valve to said second friction engaging device, and in fact possibly via a short branch conduit, the hydraulic fluid pressure supplied to said control port of the shift timing valve becomes undesirably higher than the designed due value, because of an increase of viscosity of the hydraulic fluid of the transmission when the hydraulic fluid in the transmission is cold as when the transmission has not yet fully been warmed up from the cold condition. This means that, in the cold state operation, the first friction engaging device will be disengaged too soon before the second friction engaging device is properly engaged, and this can lead to slipping of the transmission, and racing of the engine.
On the other hand, it is also important that the timing of the releasing of the first friction engaging device is not too much delayed, in relation to the engagement of the second friction engaging device, as such delayed disengagement of the first friction engaging device can lead to both the first friction engaging device and the second friction engaging device being to a large extent engaged at one and the same time, which can cause severe frictional wearing of the first and the second friction engaging devices. Further, it is necessary to be aware of the problem that often air bubbles or the like tend to become trapped into the transmission hydraulic fluid conduits at various points, and such trapped air should not be allowed seriously to deteriorate the operation of the transmission.